The invention relates to a method for performing a wireless communication in a data collection system, the system comprising a plurality of devices communicating with one another over a first data network performed by means of a single wireless channel, in which access to said channel is based on a CSMA (English acronym for Carrier Sense Multiple Access) data transmission technique.
In particular, said communication method relates to a communication system in which the devices comprise a plurality of portable data-collection units and at least one base unit, communicating over a radio frequency wireless network.
Data-collection apparatuses are known comprising data-collection units which are able to read optical information, for example barcodes or RFID tags. The data-collection unit can be grasped by an operator and is thus portable. The data-collection apparatus may further comprise a base unit arranged for receiving and supporting the portable data-collection unit when the data-collection unit is not in use, or, alternatively, for communicating with the portable data-collection unit in order to acquire the collected data from said portable unit.
Each portable unit comprises a portable battery-driven data acquisition, and possibly management, device intended for interacting with a base unit for recharging the battery and/or for exchanging data. The portable unit may be a PDT (portable data terminal) i.e. a portable terminal for the automatic or manual acquisition of data, or, in general, a portable automatic identification terminal such as a portable reader of optical information or of coded information stored in a transponder (also called in English a Radio Frequency Identification Device or “RFID tag”).
The base unit may comprise a battery-charging device that supplies batteries with which the portable unit is provided. Alternatively, if the battery-charging device is contained in the portable unit, the base unit comprises a supply circuit for the battery charger.
The portable data-collection unit can therefore assume a use configuration in which it is grasped by the user to acquire the optical information or the RFID tags and is communicated over a wireless network with the base unit, and a rest configuration, in which it is received and retained by the base unit so that the batteries can be recharged, if necessary.
Data-collection apparatuses are known, for an industrial environment or for commercial applications, that comprise a data collection system comprising a plurality of portable data-collection units and one or more base units. Each device (whether portable unit or base unit) that is part of said data collection system comprises a respective radio frequency element for communicating and exchanging data that defines a wireless data network, and over said wireless network the information, read by each portable unit in a use configuration is communicated to a respective base unit.
It should be noted that whilst each device can be in physical communication with any other device, in such systems it is known that each portable unit can communicate exclusively with a respective base, unit, in the sense that there is no provision for data exchanges between said portable units. In other words, each portable unit is associated with a respective base unit for data and command exchange.
In the industrial environment, over the years some international standards have been developed, such as, for example the Wi-Fi standard, which imposes physical requirements and data transmission specifications for using a wireless network. In particular, if single frequency is used for performing the transmission channel, the standard 802.11 provides network using a carrier sense multiple access (CSMA) and algorithms are known, such as, for example the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance), to ensure that collisions are avoided between the various, devices (whether they be portable units or base units) that are part of the wireless network.
The CSMA/CA algorithm provides for each device that wants to start a transmission listening to the channel (Listen-before-Transmit). If the channel is busy, the device generates a random number and associates a wait time therewith (known as the backoff time) that identifies the instant of time in which the device performs a transmission attempt.
A known problem of the CSMA/CA algorithm consists of the fact that as each devise of the network is asynchronous compared with others, although each device provides said random backoff time in the case of a busy channel, there is the probability that two devices transmit data simultaneously, generating interference on the transmission channel that makes correct reception of the data impossible, and above all, requires the transmission to, be repeated.
The phenomenon of interference is obviously greater the greater the throughput of the network and this typically occurs with a large number of devices present in the wireless network or with data packets of large size. In fact, as the transmission of a packet is delayed by the backoff time, the probability of two devices transmitting simultaneously increases with the throughput of the network. Further, recently, ever more sophisticated portable units have appeared on the market that are able to acquire even more evolved optical information, such as, for example, photographs. This information, when transferred to the respective base unit, generates very high throughput on the wireless network so as to increase the interference on the transmission channel so as to make the network unusable even with very few portable units communicating on the same wireless network.
The applications connected to the use of portable units in an industrial environment, or to applications of commercial type, are used increasingly so that installing complex wireless networks is very frequent in which the presence of a plurality of bases, or even of radio access points, connected in a network to one another is used to enable very wide work areas to communicate. Each base unit or each radio access point has an antenna of a given power that defines, as known, a cell, of a lesser dimension than the total work area of the wireless network and it is therefore necessary for each portable unit, which may be in a position of the work area in which several cells are located, to choose during a movement and in function of its position, with which of the base units present to be associated.
This functionality of the portable units in a wireless network is called “roaming” and typically the portable unit defines the presence of a possible base unit to use for “roaming”, detecting a physical parameter of the radio signal coming from the base.
It is known to use the intensity of the radio signal as a criterion for conducting roaming on a base unit of the set of base units present, in particular by choosing the base from which the radio signal with the greatest intensity comes. If said radio signal is above a threshold value, the base from which the signal comes is considered to be present and usable for roaming.
The choice of the intensity of the radio signal as a physical parameter for enabling a portable unit to identify whether a base unit is, in general, present, or to choose a base unit for roaming, nevertheless raises numerous problems.
In particular, if several various wireless networks are present simultaneously, a radio signal coming from a base near the portable unit could be very intense but also very noisy, in fact making communication with said base impossible. Consequently, in the presence of several base units, said portable unit could erroneously associate itself with a base unit with which communication is difficult.
The object of the present invention is to improve communication in wireless networks of known type, based on the CSMA/CA technique, in particular optimising the use of the transmission channel of a wireless network based on the CSMA/CA technique, trying to avoid transmission interference as much as possible.
Another object is to improve communication in wireless networks of known type, assessing the efficiency of the communication.
A further object is to obtain a communication method in wireless networks in which the efficiency of communication is assessed on the basis of a criterion that is independent of specific physical parameters, so as to be immune to disturbances even in the presence of very crowded known wireless networks.
In a first aspect of the invention, there is provided a method for performing wireless communication in a data-collection system, according to claim 1.
Owing to this first aspect, it is possible to devise a communication method that enables each device on the network to be synchronised with the others to minimise transmission interference, even if a signal indicating a common, shared time is not available on the wireless network. In particular, each device that wishes to transmit but cannot because the channel is already busy, calculates a variable interval of time that indicates the moment at which the channel will become free again and waits for this time before queuing for a further wait time obtained from generating a random number, as required by the CSMA/CA protocol. In this manner, all the devices that are ready for transmission will await the end of the previous transmission and simultaneously, subsequently, will wait for said further wait time. Consequently, a collision will occur only if two different devices generate the same further wait time inasmuch as each device will start to transmit in a manner that is staggered over time compared with the others.
In a second aspect of the invention, a method is provided for creating wireless communication in a data-collection system, according to claim 12.
Owing to this second aspect, it is possible to make a communication method that enables an objective assessment to be made of the presence in the network of a data-packet sender device.
In fact, by monitoring the transmission channel, processing a monitored physical signal to assess whether said packet is corrupted or not and by performing further processing in case this packet is not corrupted, it is possible to identify as being present in the network only a device, which is the sender of the monitored packet, the transmission of which is not disturbed. In fact, processing said physical signal, in addition to contributing to demodulating the physical radio signal, enables it to assess whether a packet corresponds to said physical radio signal, said physical signal being compatible therewith, or whether, on the other hand, the physical signal is spurious, or corrupted.